Piezoelectric pressure sensors are known and are widely used. Thus, they are used in pressure indexing of internal combustion engines to detect a cylinder pressure prevailing in a pressure chamber as a function of the crankshaft position or a time. Internal combustion engines include four-stroke engines and two-stroke engines such as gasoline engines, diesel engines, Wankel engines, etc. In marine diesel engines, they are used for long-term monitoring of a cylinder pressure. Piezoelectric pressure sensors are used to monitor fast pressure profiles that usually are in the range of 150 to 250 bar but including pressure peaks of 500 bar and higher if pre-ignition and engine knocking occur. However, piezoelectric pressure sensors also can be used in pressure monitoring in jet engines, gas turbines, steam turbines, steam engines, etc.
U.S. Pat. No. 3,364,368, which is hereby incorporated herein by this reference for all purposes, discloses one such piezoelectric pressure sensor that includes a membrane that protrudes directly into the pressure chamber through a bore in the pressure chamber. An edge of the membrane is welded to a housing of the piezoelectric pressure sensor. The pressure profile captured by the membrane acts onto a piezoelectric sensor that is arranged within the housing and near the membrane. The pressure profile generates electric polarization charges on the piezoelectric sensor, and these charges are transmitted electrically as signals via an electrode. The signals are proportional to the magnitudes of the pressures that constitute the pressure profile. The electrode is arranged on the piezoelectric sensor. By means of an electrical conductor, the signals are transmitted electrically from the electrode to a socket for a plug connection of a signal cable that leads to an evaluation unit. The socket is arranged on a side of the housing that faces away from the membrane.
Additionally, U.S. Pat. No. 4,675,643, which is hereby incorporated herein by this reference for all purposes, discloses a piezoresistive pressure sensor in which a sensor with piezoresistors applied thereon generates signals under the action of a pressure profile detected by a membrane. Electrodes are electrically connected to the terminals of the piezoresistors and transmit the signals into an evaluation unit via feedthroughs in the form of slide bushings from a housing of the sensor and via contact surfaces to strands of electrical conductors.
In fact, during continuous use the pressure sensor is exposed to strong engine vibrations and high temperatures of 200° C. and above. These may lead to micro friction and fretting corrosion at the contact surfaces of electrodes, terminals, plug connections and feedthroughs, thereby leading to weakening of the mechanical stability of the signal transmission. In addition, outgassing of the signal cable sheath may occur at high temperatures, and such gases cross-link locally by friction polymerization and form deposits on the contact surfaces of electrodes, terminals, plug connections and feedthroughs. Furthermore, diffusion of base metals and local build-up of oxide layers on contact surfaces of electrodes, terminals, plug connections and feedthroughs may occur at high temperatures. These effects may occur separately or in combination. As a result, the electrical resistance during signal transmission may change. Thus, the electrical contact resistance may increase from the mΩ range by several orders of magnitude into the MO range and distort the signals transmitted to the evaluation unit, thereby resulting in incorrect signal evaluations. Generally, ensuring an electrical insulation of the signal transmission at high temperatures is very important because electrical leakage currents may occur at components of the pressure sensor, and such electrical leakage currents may distort the signal transmission. Furthermore, different expansion coefficients of the components of the pressure sensor may lead to local mechanical stresses at high temperatures. Finally, the components of the pressure sensor may age prematurely at high temperatures. Such thermally induced mechanical stresses and such premature aging have detrimental effects on the service life of the pressure sensor.
A first object of the present invention is to provide a pressure sensor wherein signal distortion in signal transmission is effectively prevented. Another object of the present invention is to provide a pressure sensor wherein the signal output and the evaluation unit are mechanically stable, even with strong permanent engine vibrations. It is an additional object of the present invention to provide a pressure sensor in which the components thereof are largely devoid of mechanical stresses and premature aging even at high temperatures. Finally, it would be desirable to be able to provide a process for the cost-effective manufacture of a pressure sensor having these attributes.